For elucidating free radical-mediated signal transduction, transcriptional activation of human manganese superoxide dismutase (MnSOD) mRNA in A549 cells induced by a phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), was previoulsy examined to identify the responsive transcriptional regulator within the 5'-flanking region of the human MnSOD gene promoter. We found that TPA-responsive induction of the MnSOD gene requires binding of the MSTRE (MnSOD TPA Responsive Element) to the CREB-1/ATF-1-like transcription factor, which is activated by PKC-catalyzed phosphorylation. This induction was blocked by inhibitors of NADPH oxidase or related flavoproteins, suggesting the involvement of superoxide radical anion generation as an upstream signal for the MnSOD induction. We are currently studying the regulation of MnSOD expression in protein level by TPA treatment. TPA did not affect on the cell cycle of A549 cells, whereas H2O2 induced G2 phase arrest, indicating that superoxide is nontoxic and plays a role as a natural secondary messenger. In addition, we found that the p53 tumor suppressor is a negative regulator for the MnSOD induction. Together these results suggest that superoxide radical anion is involved in the post-translational modification of p53 that causes the induction of MnSOD.